Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/frontend/manifest-disabled.json

@@ -0,0 +1,41 @@
+{
+	"name": "miniPaint",
+	"short_name": "miniPaint",
+	"start_url": "/",
+	"display": "standalone",
+	"orientation": "landscape",
+	"background_color": "#666d6f",
+	"description": "miniPaint is free online image editor using HTML5.",
+	"icons": [
+		{
+			"src": "images/manifest/48x48.png",
+			"sizes": "48x48",
+			"type": "image/png"
+		},
+		{
+			"src": "images/manifest/72x72.png",
+			"sizes": "72x72",
+			"type": "image/png"
+		},
+		{
+			"src": "images/manifest/96x96.png",
+			"sizes": "96x96",
+			"type": "image/png"
+		},
+		{
+			"src": "images/manifest/144x144.png",
+			"sizes": "144x144",
+			"type": "image/png"
+		},
+		{
+			"src": "images/manifest/168x168.png",
+			"sizes": "168x168",
+			"type": "image/png"
+		},
+		{
+			"src": "images/manifest/192x192.png",
+			"sizes": "192x192",
+			"type": "image/png"
+		}
+	]
+}